1. Field of the Invention
The present invention relates to a projector using a lamp, such as a high-voltage mercury-vapor lamp, to be lit by alternating-current drive, and relates to a light source control method and a program for drive-controlling a lamp to be a light source of the projector.
2. Description of the Related Art
A large number of projectors generally use discharge lamps, such as high-voltage mercury lights, as light sources.
FIG. 1A to FIG. 1F are timing charts respectively illustrating drive of alternating-current-driven discharge lamps in projectors of digital light processing (DLP, registered trade mark) system. FIG. 1A illustrates timing at which each color segment of a color wheel disposed in an optical path downstream of the discharge lamp are inserted into the optical path. Here, on the color wheel, a total of four segments, which are a W filter (white, (transparent or without filter)) to enhance brightness of an image to be projected, in addition to an R (red) filter, a G (green) filter, and a B (blue) filter of three primitive colors, are division-disposed in a circle shape. The rotation of the color wheel sequentially inserts each segment into the optical path in order of R, G, W, and B in a time divisional manner.
FIG. 1B to FIG. 1E respectively exemplify drive signals A to D by rectangular waveforms in each drive condition shown in FIG. 2. In each FIGS. 1B to 1E, for example, if its drive signal level is high, a ballast of a light source lamp generates a discharge from one of two oppositely disposed electrodes of the discharge lamp to the other electrode of the discharge lamp, or if its drive signal level is low, the ballast generates a discharge from the other electrode to the one electrode, respectively.
Here, in the projector of the aforementioned DLP system has a restriction that the signal for the drive signal has to be input in synchronization with the rotation period of the color wheel.
FIG. 1F exemplifies a drive signal E that is departed from the restriction, and depicts a status in which the high and low levels are switched on the way in which the segment of the B filter on the color wheel is present on the optical path. When the discharge lamp is driven by the drive signal at such timing, as a matter of course, the projector cannot accurately project an image corresponding to an input signal.
On the other hand, there is a temperature condition for an excellent operation of the high-voltage mercury-vapor lamp, and optimizing the frequency of the alternating-current-drive achieves an appropriate temperature condition. The appropriate temperature condition assures a light-emitting efficiency and a service life, and suppresses the occurrence of flicker of the mercury-vapor lamp.
As to one technique therefore, WO 95/35645 discloses a technique, in which a current pulse is generated in a predetermined fraction of the half periods of the lamp current in order to suppress the flicker of the alternating-current-driven discharge lamp, and the current pulse has the same polarity as the lamp current and is superimposed on the lamp current in the latter part of the half period in which it is generated.
The control for the alternating-current-driven discharge lamp in the projector of the foregoing conventional DLP system has to synchronize with the segments on the color wheel, and has to make reverse the polarities of drive signal waveforms in timing with the switching timing of each segment, so that it results in limitation of the drive frequency to a certain extent.
If the color wheel consists of a total of four segments of R, G, B, and W, as shown in FIG. 2 and the frequency of an input video signal is 60 Hz adopted in an NTSC system that is one of a television system, or adopted in a generic personal computer (PC), the color wheel results in rotation with 120 Hz that is a double frequency. Therefore, the drive frequencies of the drive signals A, B, C, and E respectively shown in FIGS. 1B, 1C, 1D, and 1E become 60 Hz, 120 Hz, 180 Hz, and 240 Hz, respectively, and here, the drive frequency is resulted in the limitation of a value of an integral multiple of 60 Hz and in the limitation of a value not higher than 240 Hz as well.
Further, if the PAL system (50 Hz) that is one of the other television systems is taken into account, the projector has to find out an appropriate frequency to drive the discharge lamp on a limited condition that the frequency should be integral multiple of 50 Hz and not higher than 200 Hz, and it should be integral multiple of 60 Hz and not higher than 240 Hz, then, it is very hard to set a specific and appropriate frequency.
The technique described in the above patent document is one to change the drive waveform of the alternating-current-drive lamp within a period. If the system varying content of an image to be displayed by a micromirror device for tone control in one segment period of the wheel due to the rotation of the color wheel like the projector of the DLP system employs the above technique, it is impossible to conduct accurate tone control.